GaN epilayers nitridated initially for different times have been investigated by light scattering tomography (LST) and Raman scattering. In the LST images of the plan-view epilayers, the light scattering defects mainly distribute in ⟨112_0⟩ directions. The density of the defects is lower in epilayer nitridated initially for a longer time. The defects are considered to be the straight threading edge dislocations on {11_00} planes. The Raman shift ofE2mode is larger in the sample initially nitridated for a longer time. Our results show that the misfit between the GaN epilayer and theAl2O3substrate is more unfavorably accommodated by the threading edge dislocations in the epilayers initially nitridated for a longer time. ©1997 American Institute of Physics.